In photographic emulsion manufacturing, aqueous solutions of silver nitrate and alkali salt, hereafter referred to collectively as reactants, are delivered separately to a reaction vessel where they combine to form photosensitive silver halide crystals (silver chloride, silver bromide, silver iodide, or combinations thereof) in a precipitation reaction. During this process, metallic silver can be deposited on silver nitrate delivery equipment surfaces as a result of uncontrolled electrochemical reactions involving the silver nitrate solution and conductive equipment surfaces. The metallic silver deposits can interfere with the delivery equipment function and/or contaminate the following batch of product. The primary need for such a metallic silver cleaning process has been created as a result of the recent development and use of smaller and more complicated reactant delivery system components to control the reaction environment in which silver halide crystal growth takes place. Devices such as Apparatus and Method for Distributing Fluid described in U.S. Pat. No. 5,241,992, improve the precipitation yield as a result of improved reactant introduction hydrodynamics which are controlled by small, close-tolerance reactant distribution orifices. The presence of metallic silver fragments in such distribution orifices can severely degrade the performance of the reactant delivery system.
Common cleaning agents known to the photographic industry are usually fixing agents which dissolve silver halide from equipment surfaces. Examples of such agents are thiosulfates, thiocyanates, cyanides, sulfites, concentrated alkali halides, etc. However, these fixing agents, alone or combined, are not effective in dissolving metallic silver. Japanese Kokoku, Patent Number HEI 211990]-7451, refers to a photoprocessing equipment cleaning process using alkali halides combined with a thiosulfate solution. The combined solution dissolves silver halide which is formed in an intermediate cleaning step by reacting the alkali halide solution with silver sulfide sludge on photoprocessing equipment surfaces. The process in the aforementioned Japanese patent removes silver sulfide as an unwanted contaminant which adheres to photoprocessing equipment as a by-product of the film and paper chemical processing reactions. This aforementioned process would not, however, be effective on metallic silver because neither the alkali halide nor the thiosulfate solutions, nor their combination, would sufficiently oxidize metallic silver into a state which could be easily removed from the equipment.
Bleaching agents such as concentrated nitric acid, acidic permanganate, dichromate, persulfate, etc., are effective metallic silver solvents; however, each is disadvantaged for use in high volume cleaning applications due to human toxicity, corrosiveness, potential for sensitometric contamination of product, or environmental hazardousness.
The current invention provides a method for cleaning and preparing photographic manufacturing equipment which has metallic silver and/or silver halide deposited on the surfaces of said equipment.